ISO 6976 - Calorific Values from Composition

ISO 6976 provides methods for calculating calorific values, density, relative density, and Wobbe indices from natural gas composition.

Table of Contents

Overview

Standard: ISO 6976:2016 (and earlier editions)

Purpose: Calculate physical properties of natural gas from composition analysis without direct measurement.

Scope:

Classes:

Calculated Properties

Calorific Values

Gross Calorific Value (GCV) - Also called Higher Heating Value (HHV): \(H_s = \sum_i x_i \cdot H_{s,i}^{id}\)

Net Calorific Value (LCV) - Also called Lower Heating Value (LHV): \(H_i = \sum_i x_i \cdot H_{i,i}^{id}\)

where:

Wobbe Index

The Wobbe index indicates interchangeability of fuel gases:

\[W_s = \frac{H_s}{\sqrt{d}}\]

where $d$ is the relative density.

Compressibility Factor

Mixture compressibility at reference conditions:

\[Z_{mix} = 1 - \left(\sum_i x_i \sqrt{b_i}\right)^2\]

where $b_i$ is the summation factor for component i.

Relative Density

\[d = \frac{\rho_{gas}}{\rho_{air}} = \frac{M_{gas}}{M_{air}} \cdot \frac{Z_{air}}{Z_{gas}}\]

Reference Conditions

Temperature Options

Reference T Description
0°C (273.15 K) European standard
15°C (288.15 K) ISO standard / metric
15.55°C (60°F) US/UK standard
20°C (293.15 K) Engineering standard
25°C (298.15 K) Thermochemical standard

Reference State

Reference Type

Implementation

Constructor

// Basic constructor (uses default reference conditions)
Standard_ISO6976 standard = new Standard_ISO6976(thermoSystem);

// Full constructor with reference conditions
Standard_ISO6976 standard = new Standard_ISO6976(
    thermoSystem,                    // SystemInterface
    volumeRefT,                      // Volume reference T [°C]
    energyRefT,                      // Energy reference T [°C]
    calculationType                  // "volume", "mass", or "molar"
);

Key Methods

Method Description
calculate() Perform calculations
getValue(param) Get calculated value
getValue(param, unit) Get value with unit conversion
setReferenceState(state) Set “real” or “ideal”
setReferenceType(type) Set “volume”, “mass”, or “molar”
setVolRefT(T) Set volume reference temperature
setEnergyRefT(T) Set energy reference temperature
getAverageCarbonNumber() Average C number of mixture

Internal Data

Component properties are loaded from database table ISO6976constants:

Usage Examples

Basic Calculation

import neqsim.thermo.system.SystemSrkEos;
import neqsim.standards.gasquality.Standard_ISO6976;
import neqsim.thermodynamicoperations.ThermodynamicOperations;

// Create natural gas composition
SystemInterface gas = new SystemSrkEos(273.15 + 15, 1.01325);
gas.addComponent("methane", 0.9248);
gas.addComponent("ethane", 0.0350);
gas.addComponent("propane", 0.0098);
gas.addComponent("n-butane", 0.0022);
gas.addComponent("i-butane", 0.0034);
gas.addComponent("n-pentane", 0.0006);
gas.addComponent("i-pentane", 0.0005);
gas.addComponent("nitrogen", 0.0107);
gas.addComponent("CO2", 0.0130);
gas.setMixingRule("classic");

// Flash to ensure single phase
ThermodynamicOperations ops = new ThermodynamicOperations(gas);
ops.TPflash();

// Create ISO 6976 standard
Standard_ISO6976 iso6976 = new Standard_ISO6976(gas, 15, 15, "volume");
iso6976.setReferenceState("real");

// Calculate
iso6976.calculate();

// Get results
System.out.println("=== ISO 6976 Results (15°C/15°C, real gas) ===");
System.out.printf("GCV = %.2f kJ/m³%n", iso6976.getValue("GCV"));
System.out.printf("LCV = %.2f kJ/m³%n", iso6976.getValue("LCV"));
System.out.printf("Wobbe Index = %.2f kJ/m³%n", iso6976.getValue("SuperiorWobbeIndex"));
System.out.printf("Relative Density = %.5f%n", iso6976.getValue("RelativeDensity"));
System.out.printf("Z (compressibility) = %.6f%n", iso6976.getValue("CompressionFactor"));
System.out.printf("Molar Mass = %.4f g/mol%n", iso6976.getValue("MolarMass"));
System.out.printf("Density (real) = %.4f kg/m³%n", iso6976.getValue("DensityReal"));

Different Reference Conditions

// Standard conditions for US market (60°F)
Standard_ISO6976 us_standard = new Standard_ISO6976(gas, 15.55, 15.55, "volume");
us_standard.setReferenceState("real");
us_standard.calculate();
System.out.printf("GCV (60°F) = %.2f kJ/m³%n", us_standard.getValue("GCV"));

// European conditions (0°C metering, 25°C combustion)
Standard_ISO6976 eu_standard = new Standard_ISO6976(gas, 0, 25, "volume");
eu_standard.setReferenceState("real");
eu_standard.calculate();
System.out.printf("GCV (0°C/25°C) = %.2f kJ/m³%n", eu_standard.getValue("GCV"));

// Mass-based calorific value
Standard_ISO6976 mass_standard = new Standard_ISO6976(gas, 15, 25, "mass");
mass_standard.calculate();
System.out.printf("GCV (mass) = %.2f kJ/kg%n", mass_standard.getValue("GCV"));

Using in kWh

// Get Wobbe index in kWh/m³
double wobbeKWh = iso6976.getValue("SuperiorWobbeIndex", "kWh");
System.out.printf("Wobbe Index = %.4f kWh/m³%n", wobbeKWh);

With Pseudo-Components (TBP Fractions)

// Gas with heavy ends characterized as TBP fraction
SystemInterface richGas = new SystemSrkEos(273.15 + 15, 1.01325);
richGas.addComponent("methane", 0.85);
richGas.addComponent("ethane", 0.05);
richGas.addComponent("propane", 0.03);
richGas.addTBPfraction("C7+", 0.02, 100.0/1000.0, 0.75);  // MW=100, SG=0.75
richGas.addComponent("CO2", 0.03);
richGas.addComponent("nitrogen", 0.02);
richGas.setMixingRule("classic");

Standard_ISO6976 standard = new Standard_ISO6976(richGas, 15, 15, "volume");
standard.calculate();

// Note: TBP fractions are approximated using n-heptane properties
double gcv = standard.getValue("GCV");

Display Results Table

// Create formatted table
String[][] table = iso6976.createTable("ISO 6976 Results");

// Or display in GUI
iso6976.display();

Return Parameters

Parameter Aliases Description
SuperiorCalorificValue GCV Gross calorific value
InferiorCalorificValue LCV Net calorific value
SuperiorWobbeIndex WI Superior Wobbe index
InferiorWobbeIndex - Inferior Wobbe index
RelativeDensity - Relative density (air=1)
CompressionFactor - Compressibility factor Z
MolarMass - Average molar mass [g/mol]
DensityIdeal - Ideal gas density [kg/m³]
DensityReal - Real gas density [kg/m³]

Unit Options

For calorific values and Wobbe index:

Component Data

Supported Components

The standard includes data for:

Handling Unknown Components

Components not in the ISO 6976 database are approximated:

Component Type Approximated As
HC, TBP, plus n-heptane
alcohol, glycol methanol
Other inert (N₂)

Check for approximations:

ArrayList<String> unknowns = standard.getComponentsNotDefinedByStandard();
if (!unknowns.isEmpty()) {
    System.out.println("Warning: Components approximated: " + unknowns);
}

Versions

Standard_ISO6976 (1995 Edition)

Original implementation based on ISO 6976:1995.

Standard_ISO6976_2016 (2016 Edition)

Updated implementation with:

import neqsim.standards.gasquality.Standard_ISO6976_2016;

Standard_ISO6976_2016 iso2016 = new Standard_ISO6976_2016(gas, 15, 25, "volume");
iso2016.calculate();
double gcv = iso2016.getValue("GCV");

Validation

Reference Air Properties

Temperature Z_air (1995) Z_air (2016)
0°C 0.99941 0.999419
15°C 0.99958 0.999595
20°C 0.99963 0.999645
60°F - 0.999601

Typical Results for Dry Gas

For a typical North Sea gas (mostly methane):

References

  1. ISO 6976:2016 - Natural gas — Calculation of calorific values, density, relative density and Wobbe indices from composition
  2. ISO 6976:1995 - Natural gas — Calculation of calorific values, density and Wobbe index from composition
  3. GPA 2172 - Calculation of Gross Heating Value, Relative Density and Compressibility Factor for Natural Gas Mixtures from Compositional Analysis